As cast steel materials capable of being welded and having high toughness and high strength, SCW480, SCW550, and the like which are described in Japanese Industrial Standards have been well known. Also, in the past, steel materials shown in Patent Literatures 1 to 4 have been invented.
The steel shown in Patent Literature 1 is a pre-hardened steel for molds for plastic and has been subjected to an aging hardening heat treatment after hot working of a steel containing prescribed ingredients. In the steel shown in Patent Literature 2, high strength and high toughness are achieved by applying plastic working such as forging or rolling or high strength and high toughness are realized by cooling using a method exhibiting a high cooling effect, such as water cooling or oil cooling, in a heat treatment such as quenching, normalizing, or the like. In the steel shown in Patent Literature 3, in order to secure mechanical properties, an average cooling rate at the time of an austenitizing treatment is controlled to about 250° C./min, which is a cooling rate comparable to water cooling with regard to large-sized cast steel products having a plate thickness of about 300 mm. Moreover, in Patent Literature 4, there is disclosed a production method where a slab containing prescribed ingredients is cooled at a cooling rate of 0.5° C./second or more between solidification temperature of the slab and 1,000° C.